1. Field of the Invention
The present invention relates to an image processing method and more particularly to an image processing method used for an image reading apparatus which reads out an image of a negative film, a positive film, a reflection manuscript, and so forth.
2. Description of the Related Art
Conventionally, there is proposed a negative scanner which picks up an image on a negative film with a solid-state image sensing device such as a CCD, etc. and which image-processes image signals outputted from the CCD to display them on a monitor TV.
One example of the conventional image processing method for the negative film is represented by Japanese Patent Application Laid-Open No. 63-124665. This image processing method comprises the steps of making a histogram of all the gradation data of the image signals output from the CCD for respective R, G and B signals, finding a reference minimum value and a reference maximum value with a threshold level at 1/16 which is the maximum frequency of the histogram, and image-processing according to the reference minimum value and the reference maximum value.
However, in the above-described image processing method, a histogram is made for all the gradation data of the image signals, therefore a maximum table value of the histogram needs to be set at the same as the total number of the gradation data obtained from one negative film (5000-10000 data in one negative film), considering the case when the total number of the gradation data are concentrated in some gradation range. As a result, there is a disadvantage in that the circuit size of a counter, etc. should be large in the conventional image processing method.
And, in the above-mentioned image processing method, the gradation characteristics of the image signals output from the CCD are not corrected, so there is a disadvantage in that the image of soft and hard gradation is displayed as it is when the gradation characteristic is the soft gradation (the gradation characteristic that the frequency range is wide) and the hard gradation (the gradation characteristic that the frequency range is narrow).